Belgian police detained six people for questioning on Monday over an attack on a high-speed train as it travelled between Amsterdam and Paris in August 2015, prosecutors said. The six were later released without charge.

The new detentions add to a string of police raids on homes over the weekend after which three men were charged with plotting an attack.

Prosecutors said that a judge had ordered six home searches and detained six people in the greater Brussels area. No weapons or explosives were found.

“The investigating judge will decide on their possible detention later today,” prosecutors said in a statement.

Belgium continues to be on heightened alert three months after three suicide bombers killed 32 victims in attacks on Brussels Zaventem airport and a metro car.

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In August 2015, a man on the Thalys train that had just crossed into France from Belgium tried to open fire with an assault rifle but was overpowered by three Americans, two of them off-duty members of the US armed forces. French police termed it an Islamic extremist attack, but the gunman, Ayoub El Khazzani, maintained he wanted to commit robbery.

French authorities have linked El Khazzani to Abdelhamid Abaaoud, the ringleader of the Islamic State cell that attacked Paris and Brussels.

A Belgian police sweep over the weekend left three in jail facing terrorism charges, including relatives of the two El-Bakraoui brothers, who were among the suicide attackers who struck Brussels targets on March 22.

Belgium's Prime Minister Charles Michel said the terror level across the country would remain at the second-highest level, meaning a threat of an attack "is possible and likely".